Customize the XAML editor
In Microsoft Expression Blend, you can customize the XAML view tab in which you edit your Extensible Application Markup Language (XAML). Specifically, you can customize XAML view with regard to whether tab stops or spaces are used at the beginning of each line.
On the Tools menu, select Options, and then select Code editor in the list of components.
Next to Show settings for, select XAML editor.
To change the Font and Size properties using the associated drop-down boxes.
To change the size of the indent that is inserted when you press the TAB key, enter the indent value next to Tab size.
Choose between Insert spaces or Keep tabs, as desired.